Basic Math Wiki:
http://basicmathreview.wikispaces.com/

Being a math teacher I have never really had to use wikipedia for class. But, during graduation project reviews I have noticed that there has been a rise in the number of students who use wikipedia as a primary source on their paper. I don't think that students understand that wikipedia can be edited by anyone and that it is not always accurate. The students are just looking at it as a quick source to get the job done so they can say that they took the time to do research.